The present invention relates to a scanning device for optically scanning symbol codes, such as bar codes, comprising a laser source for producing a scanning beam, a first lens for focusing the scanning beam, a detector for detecting light backscattered by the scanned symbol codes, first and second rotatable deflection means for both transmitting and deflecting the scanning beam so as to generate a scanning pattern, and drive means for rotating the first and second deflection means around a first and second axis of rotation, respectively.
In many scanning devices, e.g. in bar code readers, multiple directional scanning is used. Because of a large longitudinal working range, laser scanners are widely used for bar code reading. To read bar codes, at least one scanning line needs to be generated to scan the bar code. However, in practice bar codes on products are situated at an arbitrary orientation. To still be able to read bar codes by using a bar code reader oriented itself in an arbitrary direction, bar code readers generating multiple-directional scanning patterns are required.
Moreover, different kinds of objects often require a different kind of scanning pattern.
In the publication "Thermal imaging systems" by J. M. Lloyd, Plenum Press, New York, 1979, pages 316-320, the general principles of the use of rotating wedges for generating scanning patterns is disclosed. However, this document only discloses passive scanning means, i.e. no additional scanning beam is generated, e.g. by a laser source, to scan an object. In this passive scanning means known per se, a light beam reflected by an object impinges upon a first one of the two rotating wedges. Before incidence, the light beam propagates along a path that makes an angle with respect to the axis of rotation of both wedges. After deflection by both wedges, the light beam propagates substantially parallel to the axis of rotation of the wedges to detection means.
EP-A-0,492,730 describes how the general principles of using two rotating wedges may be employed in active scanning devices such as bar code scanners, and what technical measures may realize a bar code scanner embodying these principles.
Both in theoretical considerations as formulated, for instance, in the publication of Lloyd mentioned above and in the practical design shown in said EP-A-0.492.730 two rotating wedges are used, the axes of rotation of both of the wedges coincide, as this was considered to be necessary to generate well defined scanning patterns. Moreover, in the arrangement shown in EP-A-0.492.730 the scanning beam impinging upon the deflection means substantially coincides with the coinciding axes of rotation of the wedges. In such an arrangement the driving shafts of the wedges will have to be hollow to allow the scanning beam to pass along the axes of rotation. Furthermore, a motor having two crown wheels of different diameters and two pinions meshing with the respective crown wheels is shown in EP-A-0,492,730. However, such an arrangement has to meet high tolerance requirements, and is not cheap to manufacture, because no standard elements are used.
WO-A-91/11290 discloses an imaging device suitable for mounting on the arm of a welding robot, close to a welding torch, in order to obtain 3-D topological data from a target surface. In the apparatus according to this international patent application a rotatable wedge prism is used which is rotatable around an axis of rotation. In use, a laser beam impinges upon the rotating wedge prism which laser beam does not coincide with the axis of rotation. Only one rotating wedge prism is used in order to generate a circular scanning pattern on an object the distance of which to the measuring device is to be established. This international patent application does not disclose or suggest that the concept of a laser beam not coinciding with one rotating wedge prism has any advantage in a scanning device in which at least two rotatable deflection means are used to generate complicated scanning patterns to scan symbol codes. The field of application of the imaging device of WO-A-91/11290 differs from the field of scanning symbol codes.